List of therms» Gum arabic

Gum arabic is one of the most important image desensitizers used in lithography. It displays two important properties: (1) it is hydrophilic (water-loving), hence, its coatings are more receptive to water than to the fatty contents of printing ink; (2) its dried coatings, although water-soluble, hold tightly to the nonimage areas of the printing surface. A good desensitizing agent should be capable of dissolving in water and leaving a microscopic water-receptive layer on the printing surface. This layer cannot be removed even with further additions of water. Many other natural and syntetic material are hydrophilic, and some are capable of adsorption on the printing surface. Some of these are gum tragacanth, cherry gum, larch gum, mesquite gum, carbohymethyl cellulose, dextrines, alginates... With the exception of CMC, none is so effective a lithographic desensitizer as gum arabic.

Gum arabic is obtained from the dried gummy substance of the acacia tree, wich grows in Arabia, Senegal, Egypt, India, and the Sudan. This particular vriety seems to have superior properties for lithography. The gum exudes naturally from the trunk and branches of the tree in the form of tears, which harden when exposed to air. These tears are separated from the bank and sand, and, after being sorted and graded for quality, are packed for shipment.

Gum arabic falls in the class of noncrystalline carbohydrates that form colloidal solutions. Chemically, gum arabic is usually considered to be a mixture of calcium, potassium, and magnesium salts of arabic acid with some free arabic acid. When nitric or phosphoric acid is added to gum arabic to make lithographic etches, most of the salts of the arabic acid are converted to free-acid form. In this condition the sollutions produce the most effective desensitization.

Pure gum arabic can be obtained from lithographic suppliers in powdered, crystalline, or liquid form. The liquid form is formulated particularly for offset lithography; it is, however, by far the most efficient for handprinting purposes as well. Research has shown that gum arabic solutions perform best for alI-round use when they are low in viscosity and high in solid content. The advantages of commercially prepared liquid gums are many. These gums are clean and free from residue, are of controlled formulation (which ensures that each batch is exactly the same), and, more important, they are nonsouring, so that stock solutions can be kept for indefinite periods of time.

Powdered and crystalline forms of gum arabic can be liquefied by mixing with water. Although band-prepared solutions of gum arabic rarely have the same consistency from one batch to another, it is well to know how they are made.

Anna Trojanowska